1. Technical Field
Embodiments relate to a starting structure, and more specifically to a starting structure adapted to protection components on silicon and especially to vertical protection components such as Shockley diodes.
2. Discussion of the Related Art
FIG. 1 generally shows the simplified diagram of a protection device which turns on when a voltage greater than a threshold is applied between its main terminals. Protection device 1 is placed between terminals T1 and T2 and comprises, or is associated with, a starting device 3. When the voltage between terminals T1 and T2 exceeds a determined threshold, a current flows in starting device 3 and starts protection device 1.
FIG. 2A shows as an example the circuit diagram of a protection device formed of a Shockley diode (PNPN diode). Such a diode is equivalent to a thyristor TH having main anode and cathode terminals A and K, the thyristor anode being connected to its gate by an avalanche diode or Zener diode Z capable of breaking down when the voltage across it exceeds a threshold value.
An issue for the monolithic forming of such devices lies in the fact that the doping levels necessary for a proper operation of the thyristor are not always compatible with the forming of doping levels capable of forming an avalanche diode Z of desired avalanche voltage. Further, it is in practice difficult to reach all the desired threshold voltage values. It is especially difficult to form devices having breakdown voltages lower than 10 volts. Indeed, to reach such voltages, junctions should be provided between very heavily-doped P- and N-type areas, which are often difficult to form and to accurately adjust.
FIG. 2B is a very simplified cross-section view showing an embodiment of the Shockley diode illustrated in the form of a circuit diagram in FIG. 2A. Only the main elements of the structure have been shown, and especially by various means currently intended to ensure its voltage behavior at the component periphery.
The Shockley diode is made in vertical form from an N-type substrate 10. P-type wells, respectively 12 and 13, are formed on the upper surface side and on the lower surface side. Currently, the P wells are formed in several steps so that the lower P well extends relatively deeply into substrate 10, to limit the thickness of the N-type region, especially for protection devices of low turn-on voltage. Indeed, if this N-type region remains too thick, the Shockley diode will have a relatively high on-state voltage drop and poor dynamic performance.
On the upper surface side, an N-type layer 15 is formed in well 12. Layer 15 is conventionally provided with emitter short-circuits 16, that is, with regions where this layer is interrupted. At the periphery of well 12 is formed a heavily-doped N-type ring 17. As desired, this ring is formed at the same time as N-type layer 15 to limit the number of manufacturing steps. The junction between N+ region 17 and upper well 12 plays the role of avalanche diode Z of FIG. 2A. When the diode corresponding to the junction between N+-type ring 17 and P-type well 12 should have a relatively low avalanche voltage (smaller than 10 volts), P-type well 12 should be relatively heavily-doped (for example, at a doping level greater than 1018 atoms/cm3). Lower well 13 is coated with an anode metallization A and the upper surface of N-type layer 15 is coated with a cathode metallization K. The upper and lower surfaces of the Shockley diode are coated with an insulating layer 19, at the locations where these surfaces should not be contacted by the anode or cathode metallizations.
The operation of this protection device is the following.
When a low positive voltage is applied between the anode and the cathode, the component is non-conductive. When this voltage exceeds the value of the breakdown voltage defined by N+ region 17 and P well 12, a current tends to flow from anode well 13 through the forward junction between this well and substrate 10, and through the diode in avalanche between N+ region 17 and P well 12, towards metallization K. This turns on PNPN thyristor 13-10-12-15.
Since N+P diode 17-12 sets the starting voltage of the component, the doping levels of these regions, and especially of P well 12, should be adjusted with a great accuracy. In practice, it is difficult to achieve breakdown voltages smaller than 10 volts and to finely adjust such breakdown voltages in this manner.